Context-dependent resilience of intertidal seagrass and venerid clams after hyposalinity stress

Abstract

Reduced salinity in estuarine areas can negatively affect intertidal seagrass meadows where clams are harvested. However, legacy effects of hyposalinity on seagrasses and infaunal clams and on their interactions have seldomly been studied. Legacy effects were examined in the intertidal seagrass Zostera noltei and juveniles of the clams Venerupis corrugata, Ruditapes decussatus and R. philippinarum. Low salinity stress was applied to mesocosms including assemblages of seagrass and the three clam species which were subsequently transplanted to two shellfish beds characterized by different environmental conditions (i.e., seawater temperature, salinity and nutrients concentration). After two months, the morphological and biochemical traits of Z. noltei and the growth and mortality of the clams were measured. Past hyposalinity stress increased clam mortality and decreased Z. noltei leaf length. The native V. corrugata was the most vulnerable to past hyposalinity, whereas the introduced clam R. philippinarum was the most resilient. The presence of clams was associated with greater N content and biomass of the above-ground parts of Z. noltei. Survival of the clams after past hyposalinity stress was greater below Z. noltei than below bare sediment, indicating persistent positive interactions following the harsh environmental conditions and during low salinity periods in the field. In the colder and more nutrient-rich site, positive interactions were more frequent and Z. noltei performed better. This supported the influence of abiotic conditions on the recovery process. Nonetheless, enhanced survival of clams below Z. noltei indicates that Z. noltei could favour the sustainability of these shellfisheries after exposure to extreme rainfall events.